This invention relates to graft polymerization of water-soluble monomers and starch. More specifically, it relates to graft polymerization under conditions such that the reaction mixture appears dry.
Water-soluble polymers, such as those containing polyacrylamide, are useful as flocculants for removal of suspended solids from water and as additives in the manufacture of paper. If we consider polyacrylamide as a representative example, prior art methods of preparation utilize the following techniques [N. M. Bikales, Polymer Sci. Technol. 2: 213 (1973)]:
1. The polymerization is carried out in water solution using any of the common initiators known in the prior art, e.g., sodium bromate-sodium sulfite.
2. The polymerization is run in an aqueous solution which is dispersed in the form of small droplets in an organic medium such as toluene.
3. The polymerization is carried out in a reaction medium in which acrylamide monomer is soluble but the polymer is not. An example of such a medium is a 40-55 percent solution of t-butyl alcohol in water, preferably in the presence of a salt.
All of these methods require an isolation and drying step. This step is often the most difficult and expensive step in the preparation of polyacrylamide and other water-soluble polymers. Polymers prepared in water solution are particularly difficult to isolate due to the high viscosities which develop as the reaction progresses.
The grafting of polyacrylamide and other water-soluble polymers onto starch is well known in the prior art, and the resulting graft copolymers find use in the same applications as the respective starch-free homopolymers. Similar to homopolymers, starch graft copolymers prepared by prior art methods all require an isolation and drying step.
Irradiation with cobalt-60 is also a commonly used method of initiating graft polymerization onto starch. Two general methods are used for cobalt-60 initiated graft polymerization: the simultaneous irradiation technique and the preirradiation technique. In the simultaneous technique, starch, in either a water solution, water dispersion, or water slurry, is mixed with a water solution of monomer and the resulting slurry, dispersion, or solution is then irradiated. In the preirradiation technique, starch is irradiated in the dry state but in the complete absence of monomer. The irradiated starch, which contains long-lived free radicals, is then added to a water solution of monomer to initiate polymerization. In both techniques, the final reaction product is a slurry, dispersion, or solution in water which is often viscous and difficult to handle. These solutions must then be dewatered and the graft copolymer isolated and dried using procedures which are time consuming and expensive.
The preparation of conventional starch derivatives using techniques where the amount of water in the reaction mixture is minimized to give an outwardly dry blend is well known in the prior art. However, in these conventional starch derivatives, the substituents which are reacted with and added to the starch backbone are of low molecular weight, e.g., acetyl, benzoyl, carboxymethyl, or aminoalkyl. Consequently, the product of the reaction is a highly substituted starch containing many substituents of low molecular weight (often one or more substituents per AGU).
Starch graft copolymers are vastly different in chemical structure from conventional starch derivatives. In a starch graft copolymer, a water solution of monomer is polymerized to give substituents which are of high molecular weight (usually more than 100,000) and which are very infrequently spaced long the starch backbone (usually more than 500 AGU separating each polymeric substituent). A technique using an outwardly dry blend of starch, monomer, and water, which is similar to that used to prepare a conventional starch derivative would not be expected to give useful products if it were used to prepare a starch graft copolymer. Since the amount of water in the system would necessarily be minimized, to give the required outwardly dry blend, the concentration of polymerizable monomer in the water solution which is blended with starch would be high. Concentrated solutions of monomer in water are known to polymerize extremely rapidly with the evolution of much heat. Such polymerizations are commonly known as "runaway" polymerizations and generally give polymers which would be crosslinked and therefore show a reduced solubility in water and would thus be of limited utility.
For a thorough discussion on starch graft polymerizations see "Block and Graft Copolymerization," Vol. 1, ed. R. J. Ceresa, John Wiley and Sons, New York, 1973, Chapters 1 and 2.